Blown film die for producing tubular film

ABSTRACT

Film die ( 10 ) for the production of tubular film ( 3 ) made of thermoplastic synthetics with at least one feed channel for the synthetic fused material, which ends in a ring-shaped discharge nozzle and is divided into sectors ( 30 ) with an interior and/or exterior cooling ring ( 1 ) that is adjusted to the diameter of the extruded synthetic tube ( 3 ), in which physical parameters of the air blown onto the extruded film tube ( 3 ) can be variably changed sector by sector,  
     whereby  
     the film die ( 10 ) is divided into sections ( 31 ), in which the temperature is individually variable, and/or additional bores ( 6 ) are arranged across the circumference in the film die, which are essentially aligned vertical to the exterior cooling ring ( 1 ), whereby air of variable temperatures can be blown from them, the physical parameters of which are variable, and that at least one control unit ( 7, 8 ) is provided, which controls or operates the various measures for the influencing of the film thickness.

[0001] The invention relates to a film die for the production of tubular film made of thermoplastic synthetics according to the generic term of claim 1.

[0002] Film dies of this type are known in various embodiments, whereby the interior cooling air supplied to and discharged through the film die in axial direction serves for the cooling of the extruded film tube and for the inflation of the film tube into a tubular film bubble, which stretches the fusible synthetic tube. However, film dies exist, in which no air is supplied and/or discharged in the film bubble during the extrusion process, but instead a predetermined static air volume is present in the film bubble.

[0003] Generally, exterior cooling rings have a ring-shaped nozzle gap that is directed toward the extruded film tube, through which cool air is blown so that the same is rapidly cooled down in order to improve the output with a so-called frost pipe that is as low as possible.

[0004] In order to control the thickness of the film tube blown into a tubular film bubble across its circumference, it is known to variably heat and/or cool the tube-shaped extruded synthetic fused material section by section in order to influence the thickness profile. For this purpose the effect is being utilized that while inflating the synthetic tube into a tubular bubble, the warmer areas are stretch formed at a stronger degree, and the cooler areas at a lesser degree. For this purpose, physical parameters of the cool air are varied sector by sector. These parameters include the temperature and the volume flow of the cool air.

[0005] It is known from DE 42 07 439 that the exterior cooling ring is divided into sectors, in which the air blown onto the extruded film tube can be heated and/or cooled. This film die has a compact construction, because the elements for the variable heating or cooling of the air blown onto the extruded film tube are directly integrated into the exterior cooling ring. Heating cartridges distributed across the circumference may be arranged on it for the heating, and Peltier elements distributed across the circumference may be arranged on it for the cooling. The variation of the physical parameters of the cool air sector by sector can also occur on or through the interior cooling ring in addition, or as an alternative.

[0006] On the other hand, film dies have been known from DE 42 36 443 A, in which heating cartridges distributed across the circumference are integrated into the film die itself, whereby these heating cartridges each can be activated in order to control the thickness of the film tube. A convertibility of the temperature of the die section by section results from this construction. However, this system has the advantage of a more lethargic control as opposed to the system with the exterior cooling ring.

[0007] Even though the film die known from DE 42 07 439 A enables a quicker and more exact control of the cool air escaping from the film die and blown onto the film tube, inaccuracies with the control of the wall thickness of the blown film tube still exist, in that interactions between the air of variable temperatures flowing from the exterior cooling ring and the ambient air occur within the area of the nozzle discharge of the exterior cooling ring. Depending on changes in ambient conditions, as the case may be, unsteady turbulences are created here, which influence the cooling results in the area of the film tube in an unpredictable manner.

[0008] It is therefore the task of the invention to embody a film die according to the generic term in such a way that the control of the wall thickness of the blown film tube can be reliably performed.

[0009] According to the invention, this task is solved based on a film die according to the generic term by means of the combination of the characteristics of claim 1.

[0010] In most cases the film die for the production of tubular film made of thermoplastic synthetics has at least one feed channel for the synthetic fused material, which ends in a ring-shaped discharge nozzle, and additionally has pipes for the feeding and discharge of interior cooling air, and an exterior ring framing the extruded synthetic tube that is divided into sectors, in which the air blown onto the extruded film tube can variably be heated and/or cooled. However, it is also possible to provide no pipes at all for the supply and discharge of interior cooling air so that the film bubble remains without any air supply in a so-called static condition.

[0011] According to the invention, the temperature of the film die is variable section by section. Usually, this convertibility is created by means of heating cartridges inside of the film die, which additionally are distributed across its circumference.

[0012] By means of these heating cartridges the temperature of the film die can be changed section by section, whereby each section is comprised of an angular range in the direction of circumference of the die.

[0013] The convertibility of the die temperature section by section, however, can also be achieved by leading air pipes through the die. These air pipes can be constructed in such a way that the air discharge and supply pipes end at the circumference of the die so that the film die itself is not influenced by these airstreams. If such pipes are used, the change of the die temperature occurs due to the fact that the physical parameters in the air, which flows through these pipes, are changed. These parameters include especially the temperature and the volume of the air per time unit. As a general rule, an air pipe will be provided per section in order to be able to vary the temperature of the die section by section. Of course, other possibilities of influencing the die temperature are also conceivable. It is important for the purposes of this invention, however, that this change can occur section by section.

[0014] Furthermore it is not necessary to limit the influence of the said measures of the temperature influencing on the area of the die, which is located in radial direction outside of the film tube, or of the nozzle ring, respectively. The change of the die temperature performed section by section can also be performed in sectors within the nozzle ring, or by sectors inside and outside of the nozzle ring.

[0015] As an addition, or as an alternative, additional bores may be arranged across the circumference in the film die, which are essentially aligned vertical to the exterior cooling ring, whereby air of variable temperatures can be blown from them. According to the invention, a first control unit can be provided for the heating cartridges and/or cooling elements in the exterior cooling ring, and a second control unit for the heating cartridges, or for the control of the volume flow, or for the temperature of the air flowing from the additional bores in the film die.

[0016] According to an advantageous embodiment of the invention, heating cartridges and/or cooling elements are arranged in the exterior cooling ring across the circumference.

[0017] The control units can be activated depending on the signal of a thickness measurement device for the wall thickness of the film bubble.

[0018] Preferably, a dependent control is performed in such a way that initially the heating cartridges and/or cooling elements of the exterior cooling ring, and the more lethargic heating cartridges next in line are activated within the film die.

[0019] Surprisingly, it has been shown that the overlap of the cooling of the film bubble across the exterior ring by means of the more lethargic heating cartridges admitted into the film die itself enables a more exact control of the wall thickness of the film bubble.

[0020] Additional details and advantages of the invention are explained in more detail in the embodiment examples illustrated in the drawing. The figures show:

[0021]FIG. 1: An embodiment example of an inventive film die, which is illustrated at a vertical section.

[0022]FIG. 2: A drawing of the position of the sectors of an exterior cooling ring, and of the position of the heating cartridges, which are arranged on the circumference of a die.

[0023] The synthetic fused material 12 is fed to the film die 10 from the schematically arranged extruder via an adapter fitting 14. The synthetic fused material is then directed to a ring manifold channel 16 for its distribution, which then ends into the ring-shaped discharge nozzle. The partially sectional film tube 3 is discharged from the ring-shaped nozzle gap, which is embodied of two die rings. A cooling ring 1 is located on the exterior die ring, which directs cooling air onto the extruded tubular film 3 directly above the die ring.

[0024] The cooling air being fed is mutually delivered by means of a not illustrated blower for interior and exterior cooling, and fed to a connecting pipe 18 to the exterior cooling ring 1, or via a pipe 20 to the interior cooling ring 22, the discharge gap 24 of which is preferably tilted toward the bottom, or is simply arranged to blow radial toward the exterior so that the sharply bundled air jet hits the interior side of the film tube at about the same height as that of the exterior air jet. The general construction of the film die as seen in the figure is known from DE 1 96 09 62 A, or from DE 42 07 439 A, to which reference is made for purposes of simplifying any additional description.

[0025] Heating rods that are evenly distributed across the circumference are arranged in the exterior cooling ring 1 of the film die according to the invention, which protrude radial from the exterior to the interior. As is obvious in the figure, the heating rods 4 are arranged in the ring-shaped exterior wall of the exterior cooling ring 1. The heating rods 4 are equipped with heating cartridges, which can be activated independently of one another.

[0026] Heating cartridges 5 evenly distributed across the circumference, which can be activated independently of one another, are arranged in the film die 10 next to the exterior cooling ring 1 with the heating rods 4 in the area of the ring-shaped discharge nozzle.

[0027] In the upper area of the film tube bubble, i.e., in the area after the frost pipe, a thickness measurement unit 9 is arranged for the measurement of the thickness of the film tube. The corresponding measurement readings are fed to a data processing and control unit 8 for the heating rods 4 in the exterior cooling ring. The readings are then transmitted from this data processing and control unit 8 to a data processing and control unit 7 for the heating cartridges 5 in the film die.

[0028] The embodiment illustrated additionally provides air pipes 6 in the film die, which are aligned essentially vertical to the exterior cooling ring, as illustrated in the figure. The air pipes 6 distributed across the circumference are also integrated in the film die 10. Variably heated or cooled air, which hits the cooling air stream in the manner illustrated, which is discharged from the nozzle gap of the exterior cooling ring 1, can then be fed through the air pipes 6 in a controlled manner. Here it is also possible, for example, to vary the volume flow of the air, which is fed through the air pipes 6.

[0029]FIG. 2 shows a drawing of the position of the heating cartridges of the exterior cooling ring 4, and of the die 5 in an embodiment example of the invention. Sections 30 are assigned to the latter 5, within which a variation of the temperature of the heating cartridge 5 significantly influences the temperature, and therefore the viscosity of the fused material. One of these sections 30 is illustrated at a hatching, which extends laterally at a 45° angle from the bottom left to the top right. Analogous to the sections 30, sectors 31 are shown, within which the extruded film tube is influenced primarily by the heating effect of the assigned heating cartridge 4. One of these sectors 31 is highlighted by a hatching, which extends at a 45° angle from the top left to the bottom right. In dies according to the invention, resulting sectors 32 are created from the sectors 31, and from the sections of the die 30. In the shown embodiment example, the die is arranged to the exterior cooling ring 1 in circumference direction in such a way that the resulting sectors overlap a smaller angle than each of the sections 30 and sectors 31 individually.

[0030] A resulting sector 32 is highlighted in FIG. 2 by means of the overlapping of both above mentioned hatchings.

[0031] Such an arrangement enables a higher resolution with the control of the film's viscosity. Reference List 1 Cooling ring 2 3 Film tube 4 Heating rods 5 Heating cartridges 6 Air pipes 7 8 Control unit 9 Thickness measurement device 10 Film die 11 12 Synthetic fused material 13 14 Adapter fitting 15 16 Ring manifold channel 17 18 Connecting pipe 19 20 Pipe 21 22 Interior cooling ring 23 24 25 30 Sections of the die 31 Sectors of the cooling rings 32 Resulting sectors 

1. Film die (10) for the production of tubular film (3) made of thermoplastic synthetics with at least one feed channel for the synthetic fused material, which ends in a ring-shaped discharge nozzle and is divided into sectors (30) with an interior and/or exterior cooling ring (1) that is adjusted to the diameter of the extruded synthetic tube (3), in which physical parameters of the air blown onto the extruded film tube (3) can be variably changed sector by sector, whereby the film die (10) is divided into sections (31), in which the temperature is individually variable, and/or additional bores (6) are arranged across the circumference in the film die, which are essentially aligned vertical to the exterior cooling ring (1), whereby air of variable temperatures can be blown from them, the physical parameters of which are variable, and that at least one control unit (7, 8) is provided, which controls or operates the various measures for the influencing of the film thickness.
 2. Film die according to claim 1, characterized in that heating cartridges (5) and/or cooling elements are arranged in the exterior cooling ring across the circumference.
 3. Film die according to claims 1 or 2, characterized in that at least one control unit (7, 8) can be activated depending on the signal of a thickness measurement device (9) for the wall thickness of the film bubble (3).
 4. Film die according to one of the claims 1 to 3, characterized in that in order to control the wall thickness of the film bubble, initially the means (4) for the influencing of the physical parameters of the air of the cooling ring (1), and supplemental to that the means (5) for the change of the temperature of the sections (30) of the film die (10) are activated.
 5. Film die according to one of the previous claims, characterized in that at least the air temperature, which is blown from the interior and/or exterior cooling ring (1) onto the extruded film tube (3), belongs to those physical parameters that can be changed sector by sector.
 6. Film die according to one of the previous claims, characterized in that at least the volume of the air, which is blown per time unit from the interior and/or exterior cooling ring (1) onto the extruded film tube (3), belongs to those parameters that can be changed sector by sector.
 7. Film die according to one of the previous claims, characterized in that the sections (30) of the film die (10), in which the temperature can be changed individually, the sectors (31), in which the physical parameters of the air, which is blown from the interior and/or exterior cooling ring (1) onto the extruded film tube, can be changed, are arranged in the circumference direction of the extruded film tube in relation to each in such a way that resulting influencing ranges of the sections (30) and sectors (31) are created, which overlap a smaller angle in circumference direction of the film tube, than the sections, or sectors individually.
 8. Film die according to one of the previous claims, characterized in that both the exterior cooling ring (1) and the interior cooling ring is divided into sectors (31), within which the physical parameters of the air, which is blown from the interior or exterior cooling ring (1) onto the extruded film tube (3), can be varied sector by sector, and that the sectors (31) of the one ring (1) are arranged in relation to the sectors of the other ring in the circumference direction of the extruded film tube (3) in such a way that the resulting influencing ranges (32) of the sectors of both rings overlap smaller angles in the circumference direction of the film tube, than the sectors (31) of both rings (1) individually.
 9. Method for the production of tubular film (3) made of thermoplastic synthetics with a film die (10) with at least one feed channel for the synthetic fused material, which ends in a ring-shaped discharge nozzle and is divided into sectors (30) with an interior and/or exterior cooling ring (1) that is adjusted to the diameter of the extruded synthetic tube (3), in which physical parameters of the air blown onto the extruded film tube (3) can be variably changed sector by sector, whereby the film die (10) is divided into sections (31), in which the temperature is individually variable, and/or additional bores are arranged across the circumference in the film die, which are essentially aligned vertical to the exterior cooling ring (1), whereby air of variable temperatures can be blown from them, the physical parameters of which are variable, and that at least one control unit (8, 9) is provided, which controls or operates the various measures for the influencing of the film thickness. 